Hydroxyl group-containing vinyl ether (co)polymers are excellent in adhesion with a substrate and processability via a cross-linking reaction with various resins, and are useful as resin modifiers, paint components, ink components, adhesive components, metal recovery resins, hygroscopic materials, compatibilizers, surfactants, or the like. For instance, Patent Document 1 discloses, as a vinyl ether copolymer suitable for an adhesive composition, a random copolymer composed of a propyl vinyl ether unit and diethylene glycol vinyl ether unit.
Further, as shown in Non-patent Document 1, it has been known that hydroxyl group-containing vinyl ether (co)polymers exhibit responsiveness to thermal stimulation. Thermal stimulation responsive polymers have the characteristics of reversibly altering the solubility to water, that is, hydrophilicity and hydrophobicity, below and above a certain temperature; and thus use of this in a member component allows for switching the hydrophilicity and hydrophobicity of the member component by temperature. Further, by making the copolymer as one with vinyl ether having various functional groups, it is expected to obtain materials capable of controlling various functions depending on the temperature. Copolymers with hydrophobic monomers, in particular, greatly affect interaction with hydrophobic substances and are suitable to be used in a system responsive to temperature stimulation.
Incidentally, cationic polymerization is known to progress because the vinyl ether monomer is a monomer having an electron-donating substituent as its property. However, because the hydroxyl group induces a termination reaction for a cationic polymerization catalyst, in cases where a vinyl ether (co)polymer containing the hydroxyl group is obtained by the cationic polymerization, monomers with the hydroxyl group being protected are polymerized and thereafter the step of deprotection is carried out (see, for example, Patent Document 1 and Non-patent Document 1). Further, the cationic polymerization is usually carried out under low temperatures equal to or less than 0° C.; and, because of heat of the reaction, it is not easy to control the temperature in an industrial scale. In addition, the reaction is required to be carried out in an anhydrous condition and inert gas. It has therefore been difficult to industrially produce the hydroxyl group-containing vinyl ether (co)polymer via the cationic polymerization at low cost in an efficient fashion.
Meanwhile, vinyl ether is known to exhibit alternating copolymerizability with a strong electron accepting monomer; and an alternating copolymer in which the vinyl ether is introduced appropriately 50 mol% can be readily obtained, for example, by subjecting the vinyl ether and fluoroolefin to radical polymerization at a molar ratio of 1:1. That is, according to a method of alternating copolymerization with fluoroolefin, a copolymer containing a hydroxyl group can be obtained by using hydroxyl group-containing vinyl ether as is in polymerization (see, for example, Patent Documents 2 and 3).
However, because the vinyl ether exhibits, when used alone, a low radical polymerization reactivity, radical polymerization hardly ever progressed and an oligomer cannot be obtained even if the reaction takes place (see, for example, Non-patent Document 2). Due to this, it has been difficult to obtain a vinyl ether (co)polymer containing a hydroxyl group by the radical polymerization using the vinyl ether alone as a monomer.
On the contrary, Non-patent Document 3 reports that, with regard to oligoethylene glycol methyl vinyl ether, the electron density of radical chain propagation carbon can be lowered by carrying out polymerization in water, ethanol, or a water/ethanol mixed solvent in the presence of a water-soluble radical polymerization initiator containing an amidino group, thereby allowing radical polymerization to progress. It is however shown that, in cases where 2,2′-azobisisobutyronitrile (AIBN) which is hardly soluble in water is used as a polymerization initiator, the reaction hardly progresses. Further, the monomers used in the reaction are all hydrophilic, the polymerization initiator is water soluble, and the solvent used is also water or one that contains water. Thus, such a reaction is disadvantageous for introducing hydrophobic monomers.